The present invention relates generally to a method of drying crystalline material. More in particular, the present invention relates to a fluidized method for drying hydrated crystalline magnesium sulfate.
In drying hydrated crystalline magnesium sulfate, particularly Epsom salt (MgSO.sub.4.7H.sub.2 O), it is desirable to remove moisture from the surface of the crystalline material without removing waters of hydration. It is well known that surface moisture can be removed from hydrated crystalline salts by the use of a continuous rotary dryer. The operation and design of such a dryer are described in detail by R. H. Perry and C. H. Chilton, in Chemical Engineers' Handbook, 5th ed., McGraw-Hill Book Co., New York, N.Y., (1973), pp 20-30 through 20-45.
While such conventional rotary dryers can, in some situations, be satisfactory for drying, they suffer from a number of disadvantages. For example, rotary dryers consume large amounts of energy and can be thermally inefficient. Moreover, they can cause excessive particle size degradation.
Many of the disadvantages associated with conventional rotary dryers can be overcome by utilizing fluidized bed technology. The general properties and advantages of fluidized systems are well known. They are described, for example, by R. H. Perry and C. H. Chilton, in Chemical Engineers' Handbook, 5th ed., McGraw-Hill Book Co., New York, N.Y., 1973, pp 20-67 through 20-74. Most of the applications of fluidized bed technology have been to catalytic systems, calcining, or bulk drying of materials such as sand and coal, rather than to drying chemical compounds.
The theory and practice of fluidized bed dryers for chemical compounds are not well established. Information is scanty on the design and performance of fluidized systems for the thorough drying of conventional chemical compounds. (G. Nonhebel and A. A. H. Moss, "Drying of Solids in the Chemical Industry", CRC Press, Cleveland, Ohio, (1971) pp 205/210). As a result, it is not even generally known which materials may be dried successfully in a fluidized system and which may not. (V. Vanecek, M. Markvart, and R. Drbohlav, "Fluidized Bed Drying", Leonard Hill Ltd., London (1966), p. 7).
Surveys of the materials which are known to have been successfully dried in fluidized bed dryers are reported in "Fluidized Bed Drying", ibid, at pp 145-155, and in "Drying of Solids in the Chemical Industry", ibid, at p. 211. However, crystalline material containing both waters of hydration and a small amount of surface moisture, has not heretofore been dried in a fluidized bed dryer to selectively remove only the surface moisture content from the material. Moreover, it would be unexpected that such selective dehydration of crystalline magnesium sulfate would be possible in a fluidized system, since it is commonly thought that moist crystalline material which tends to agglomerate cannot be processed in a fluidized system.